Acid treatments are used to stimulate the production of hydrocarbons from hydrocarbon-bearing formations such as limestone and dolomite by increasing the permeability thereof. An aqueous acid solution is pumped downhole through a borehole and into the hydrocarbon-bearing formation. The acid rapidly diffuses, reacts with and penetrates the hydrocarbon-bearing formation, increasing the permeability of the hydrocarbon-bearing formation and stimulating hydrocarbon production.
Due to the rapid diffusion of the acid, most of the acid will be spent near the borehole where the acid is introduced downhole, without penetrating deeply and/or uniformly into the hydrocarbon-bearing formation. The resulting increase in hydrocarbon production is limited by the areas of the hydrocarbon-bearing formation that are not reached by the unspent acid.
Acid-in-oil emulsions, which have an external oil phase and an internal acid phase, are used to delay the acid reaction rate of the acid treatment. The external oil phase provides a barrier to acid diffusion and allows for the acid to be delivered to greater depths and/or more uniformly within the hydrocarbon-bearing formation before the acid reacts with the materials of the hydrocarbon-bearing formation. The delayed reaction of the acid treatment results in improved stimulation of the production of hydrocarbons from hydrocarbon-bearing formations.
Commercially available acid-in-oil emulsions utilize an oil phase such as diesel, crude oil, kerosene or naptha, which are considered priority pollutants. In addition, commercially available acid-in-oil emulsions utilize emulsifiers or other components which also contain priority pollutants. Priority pollutants are a specific set of chemical pollutants classified by the U.S. Environmental Protection Agency (EPA) to regulate their use in treating hydrocarbon-bearing formations. A complete list of priority pollutants identified and regulated by the EPA is provided at http://water.epa.gov/scitech/methods/cwa/pollutants.cfm and can also be found in Appendix A to 40 CFR Part 423.
It is therefore desirable to provide an alternative to commercially available acid-in-oil emulsions which solves one or more of the aforementioned problems associated with these commercially available acid-in-oil emulsion compositions.